The present invention concerns an electronic watch providing barometer or altimeter reading based on ambient pressure, including:
a case the interior of which is divided by separating means into a sealed part and a pressure chamber which communicates with the exterior of the case so as to receive the ambient pressure,
an electric power source,
a pressure sensor having one side exposed to the pressure prevailing in the pressure chamber,
display means for providing time indication and said barometer or altimeter reading, and
a printed circuit board disposed in the sealed part of the case and provided with timekeeping circuits and electronic circuits arranged to receive and process an output signal from the pressure sensor and to control the barometer or altimeter reading display means.
The invention also concerns a method for manufacturing such a watch.
Electronic watches of this type are known, in particular from U.S. Pat. No. 4,835,716 or from European Patent No. 640 896 which provides the analogue display of the atmospheric pressure, the tendency of such pressure and the altitude by means of hands in connection with the watch dial.
In this type of barometer and/or altimeter watch, the implantation of the pressure sensor poses problems as regards the space requirement, the watch tightness, electric connections and the internal deformations due to the pressure differences between the sealed part and the pressure chamber in which ambient pressure prevails, i.e. atmospheric pressure, to which hydrostatic pressure is added if the watch is immersed in water.
The case of the first electronic barometer and/or altimeter watches included a lateral appendix specially designed to contain the pressure sensor and having orifices communicating with the outside environment. See for example U.S. Pat. No. 4,783,772 and European Patent No. 345 929. This solution had the advantage of keeping the sealing system of conventional watch cases, but this appendix was unattractive and sometimes inconvenient. This thus lead to the pressure sensor being implanted inside a watch case of ordinary shape, which requires a pressure chamber in case.
This results in additional difficulties to assure the sealing and resistance to pressure prevailing within the case.
For example in the watch disclosed in European Patent No. 640 896, the pressure sensor is housed in the case in proximity to the middle part, under a plate of the clockwork movement against which it is held. A small pressure chamber is arranged between the top of the sensor and the plate and communicates with the exterior via a channel which passes through the plate and the middle part and opens out under a rotating bezel. A cover arranged between the back cover of the case and the sensor carries electric connections connecting the latter to an integrated circuit located beside the sensor and processing the signal to provide barometer and/or altimeter readings. Additional electric connections are necessary to connect this integrated circuit to the other electric circuits of the watch, in particular to those which control the display.
This known construction takes a considerable amount of space as regards height and requires special arrangements to assure sealing around the pressure chamber and around the channel connecting the latter to the outside.
Further, there is a problem of variation in the sensor output signal as a function of temperature. This variation is different from one sensor of the same type to the next. For a variation of 10xc2x0 C., the altimeter readings provided with piezo-electric sensors currently used, which are relatively inexpensive, may vary by up to approximately. 100 m in altitude.
The Applicant has imagined calibrating each sensor as a function of temperature variations, calculating the signal calibrating parameters and storing them in the electronic circuits associated with the sensor, but such a process would be excessively expensive in the industrial manufacture of a watch according to European Patent No. 640 896 because it could only be applied once the sensor and the associated electronic circuits are assembled, i.e. after assembly thereof in the case.
In European Patent No. 670 532, an internal bottom is provided separating the interior of the watch case into a sealed part, which contains the watch movement, and a pressure chamber which is located between the internal bottom and an outer bottom which is pierced with several holes. The internal bottom carries a pressure sensor which is electrically connected to the electronic movement control by a flexible contact tongue allowing any bending of the internal bottom due to pressure to be absorbed. However, this construction has a relatively large thickness and does not resolve the aforementioned calibration problem.
The object of the present invention is to avoid the aforementioned drawbacks and it concerns a watch, in particular a wristwatch, allowing individual calibration of the readings provided by an inexpensive pressure sensor, owing to a suitable construction and an economically acceptable manufacturing method.
An additional object of the invention consists in arranging the watch so as to guarantee proper sealing and to avoid problems due to deformations resulting from variations in ambient pressure in the pressure chamber.
According to a first aspect of the invention, there is provided an electronic watch of the type defined hereinbefore, characterised in that the pressure sensor is secured to the printed circuit board, to which it is directly connected by electric connections.
These features allow the pressure sensor to be associated with the electronic circuits intended to process its output signal, at an early stage of manufacture. Thus, temperature measuring means may be installed on the printed circuit board and the electronic circuits may include a non volatile memory wherein individual calibrating parameters of the pressure sensor can be stored, and these parameters can be determined and stored in the memory before the instrument is assembled, as will be explained hereinafter.
However, it should be noted that the invention may also be implemented in an instrument which has no temperature measuring means, thus, also without using a memory for individual pressure sensor calibrating parameters, either because inferior pressure measuring precision is accepted, or because a pressure sensor which is temperature compensated or which has a low temperature related drift becomes available at an acceptable price.
Preferably, the separating means include a separating wall connected to the case in a sealed manner along its periphery and having an opening in which said sensor is placed.
This allows the case interior to be conveniently divided into a sealed part and a pressure chamber which may have any shape and size. This wall bears the effect of the external pressure in the pressure chamber and thus protects the plate carrying the display members and, if necessary, the clockwork movement in the sealed part. The separating wall may easily be connected to the case in a sealed manner along its periphery, like a double bottom, and the pressure chamber extending between this double bottom and the back cover of the case may advantageously contain an electro-acoustic transducer arranged to transmit sounds in this chamber which communicates with the outside.
The sensor used is preferably a pressure and temperature sensor of the piezo-resistive type including resistors connected in a Wheatstone bridge in which the bridge resistance varies only as a function of temperature, while variations in pressure create an unbalance in the bridge.
Another aspect of the invention concerns a method for manufacturing a watch as defined above, wherein said electronic circuits include a non volatile memory intended for storing the individual pressure sensor calibrating parameters, the method including the successive steps of:
a) manufacturing the printed circuit board and mounting at least the pressure sensor and said electronic circuits on this board to form a sub-assembly;
b) calibrating said sub-assembly in different temperature and pressure conditions and determining sensor calibrating parameters as a function of such conditions;
c) storing the calibrating parameters in the non volatile memory of the electronic circuits;
d) if necessary, completing the sub-assembly with other components which have to be carried by the printed circuit board; and
e) mounting said sub-assembly and the other watch components in the case.